Integrate hspec in Gammapy

The hspec code was added in gammapy.hspec In #264 . There’s a lot of work to do to properly integrate this in Sherpa directly or in gammapy.spectrum. Here’s an incomplete list:

• Install Sherpa on travis-ci so that it becomes possible to test this
• Make the command line gammapy-sherpa-hspec tool work again. -> this was replaced by two gammapy-spectrum-* command line tools.
• Add a basic test that runs an example analysis.
• Expose the EBL absorption code as separate classes or functions, download the files from here.
• Make a proposal how to refactor this code, i.e. which functions / classes we want and where they should go (gammapy.spectrum, naima, Sherpa directly)
• Re-factor piece by piece (this can partly be done just by improving the code in gammapy.hspec, e.g. using the Sherpa class-based API instead of the procedural API to avoid side effects via the global session object.

cc @registerrier @ignasi-reichardt @kingj90 @zblz

[cdeil]

@joleroi @registerrier - I’m assigning this issue to you and to the PyGamma1 milestone, i.e. let’s try and finish this up this week.

Please also have a look at https://github.com/zblz/naima/pull/119.

At the moment Naima is more advanced for spectra. Probably the EBL absorption code should be contributed there and then we import stuff from Naima where useful in Gammapy?

[cdeil]

gammapy/hspec was removed in #593. I’m closing this issue now.

Please open new issues to discuss e.g. EBL absorption or other related topics (and reference the discussion from this issue if you like).